The present invention relates to CMOS (complementary metal-oxide silicon) image sensors, and in particular to charge coupled devices (CCDs).
There are two primary types of CMOS active pixel image sensors. The first uses a photodiode. The second uses a photogate active pixel sensor (APS). The photogate provides high conversion gain and high performance. However, the gate material itself can absorb much of the incident light, resulting in a low quantum efficiency (QE). The QE is especially bad for short wavelength light (e.g., blue light), resulting in poor image color quality.
Modern CMOS fabrication technologies are scaled to deep submicron sizes (less than 0.35 micron feature sizes). For such small sizes, polycide is typically used as a gate material. Unfortunately, polycide will degrade the QE even further due to its strong light absorption. The result of a low QE for the sensing element is that the image must be otherwise magnified, such as by using larger and more costly aperture lenses in order to provide the same quality output signal.
One approach to limiting the absorption of incident light by the gate material is to provide multiple rectangular holes in the rectangular polycide or polysilicon gate. Thus, the light is able to penetrate the gate through the holes. A disadvantage of this approach is that some of the charge underlying the holes will remain in the holes as residual charge after a charge transfer action. This can result in image smear or signal lag. The reason for the residual charge appears to be due to the higher potential under the holes caused by less effective gate modulation.
In a typical architecture, the photogate is connected to a charge coupled device stage which includes a floating diffusion as a sense node, an optional transfer gate, and the photogate. This allows the accumulated charge under the photogate to be transferred to the sensing node when the appropriate voltage is applied to the photogate. The charge transferred is then converted into a voltage by a transistor and is further read out by proper circuitry.
The charge transferred is then converted into a voltage by a transistor and is further read out by proper circuitry.
The present invention provides an image sensor integrated circuit with a pixel cell having photogates wherein each photogate has a number of gaps which allow light to penetrate to the substrate. The gaps are open in the direction of the floating diffusion sense node, in order to minimize the trapping of charges.
In a preferred embodiment, the photogate has a comb structure, with the tines of the comb extending toward the floating diffusion. Preferably, the tines or fingers are wider than the gaps. In a preferred embodiment, the gaps are no wider than 3 microns.
In the present invention, although the potential across the whole photogate area is not uniform, the potential along the charge transferring direction is uniform. Thus, the trapping and loss of charge is minimized. Another advantage of the invention is that a higher QE is provided due to the ability to provide more open area than a scheme using holes as in the prior art.
For a further understanding of the nature and advantages of the invention, reference should be made to the following description taken in conjunction with the accompanying drawings.